Airborne observations and modeling of springtime stratosphere-to-troposphere transport over California

Stratosphere-to-troposphere transport (STT) results in air masses of stratospheric origin intruding into the free troposphere. Once in the free troposphere, ozone (O-3)-rich stratospheric air can be transported and mixed with tropospheric air masses, contributing to the tropospheric O-3 budget. Evidence of STT can be identified based on the differences in the trace gas composition of the two regions. Because O-3 is present in such large quantities in the stratosphere compared to the troposphere, it is frequently used as a tracer for STT events. This work reports on airborne in situ measurements of O-3 and other trace gases during two STT events observed over California, USA. The first, on 14 May 2012, was associated with a cutoff low, and the second, on 5 June 2012, occurred during a post-trough, building ridge event. In each STT event, airborne measurements identified high O-3 within the stratospheric intrusion, which were observed as low as 3 km above sea level. During both events the stratospheric air mass was characterized by elevated O-3 mixing ratios and reduced carbon dioxide (CO2) and water vapor. The reproducible observation of reduced CO2 within the stratospheric air mass supports the use of non-conventional tracers as an additional method for detecting STT. A detailed meteorological analysis of each STT event is presented, and observations are interpreted with the Realtime Air Quality Modeling System (RAQMS). The implications of the two STT events are discussed in terms of the impact on the total tropospheric O-3 budget and the impact on air quality and policy-making.